Vane pumps are used for supplying fluid to a hydraulic motor of a power steering mechanism. The vane pump includes a rotor that is rotatable within a cam ring. The rotor of the pump includes a plurality of circumferentially spaced grooves. A vane is carried in each groove. The vanes extend radially outwardly from the grooves of the rotor toward a surface of the cam ring. Pumping pockets are formed between adjacent vanes. The pumping pockets receive fluid from an inlet port and deliver fluid to a discharge port of the pump.
When the pump is at rest, i.e., the rotor is stationary relative to the cam ring, the vanes may move radially inwardly into the grooves of the rotor and away from the surface of the cam ring. When the rotor begins to rotate and one or more of the vanes of the pump are in a radially inward position, the amount of fluid discharged from the pump is low relative to pump operation with all of the vanes extended radially outwardly toward the surface of the cam ring.
A hydraulic power steering mechanism requires a minimum flow rate of fluid from the pump for proper operation. When the flow rate is below the minimum value, the power steering mechanism may be non-responsive to inputs requesting power steering assistance.
A vane pump generally cannot provide a fluid flow sufficient to reach the minimum flow rate until all of the vanes of the pump move radially outwardly toward the cam ring surface. Thus, the power steering mechanism may be not sufficiently responsive from pump start-up until all of the vanes are positioned radially outward toward the cam surface.
Upon start-up of the vehicle, the vane pump is rotated from a rest position to an angular velocity that is equal to the engine idle speed. For example, some commercial truck engines idle at a speed of between 600 and 750 rpm.
In some vane pumps used for supplying fluid to a power steering mechanism, all of the vanes may not move radially outward toward the cam ring until the pump reaches an angular velocity that is greater than the vehicle engine's idle speed. For example, in some pumps all of the vanes do not extend radially outwardly toward the cam ring until the rotor of the pump rotates at approximately 900 rpm. Thus, the power steering mechanism in the vehicle having one of these pumps may not be sufficiently responsive until the engine speed is increased to about 900 rpm. It is desirable to increase the responsiveness of the hydraulic power steering mechanism and to provide a pump in which all of the vanes move radially outward toward the cam ring at a pump speed that is well below the vehicle engine's idle speed.